1. Field of the Invention
The present invention relates to the deposition, by plasma-polymerization, of a protective layer, film or overcoat onto the face surfaces of substrates shaped from plastic materials.
2. Description of the Prior Art
It is known to this art to coat plastic substrates with a thin barrier layer which is highly impermeable to gases (oxygen, water vapor) via plasma techniques.
Thus, plastic substrates are known that have been coated with a layer of "silica" type, the advantage of which in relation to the existing gas-barrier materials being that the films exhibit high gas-barrier characteristics which are equal to or superior to impermeable barrier materials such as PVDC (polyvinylidene dichloride) or EVOH (polyethylene/vinyl alcohol). These withstand subsequent heat treatments such as sterilization and are transparent to microwave radiation (because of their high electrical resistivity). Such high-barrier films find application in the food packaging industry, where requirements in respect of marked impermeability to gases (O.sub.2, water vapor, CO.sub.2) are increasingly strict.
Other than their impermeability or gas-barrier properties, these plasma deposits are also useful for the protection of plastics against scratching and abrasion, especially by reason of the fact that they are crosslinked, which imparts considerable hardness thereto.
The deposition of thin layers by plasma polymerization comprehends forming a film at the face surface of a substrate from polymerizable species introduced into a plasma-forming gas (for example saturated or unsaturated hydrocarbons). In a plasma polymerization process the starting material monomers interact with energetic species (electrons, ions, photons) in the gas phase, effecting the breaking of chemical bonds and thus creating free radicals. These free radicals created in the gas phase are adsorbed at the surface of the substrate and bond together and "polymerize."
The coating or film thus formed is deemed a plasma film or a plasma deposit.
Another technique for the deposition of protective layers that provides good results is based on varnishes. However, the advantages of the plasma process, when compared with the varnish technique, are numerous. Thus, the plasma process is a dry route, in contrast to the processes employing varnishes, known as wet-route processes, in which the monomer is used in combination with one or more solvents for reasons of viscosity. These processes require strict conditions for application, especially dust-free rooms at controlled temperature and humidity.
This "clean and non-contaminating deposition" aspect of the plasma process is significant and will become increasingly so in the future, taking account of increasingly stringent government regulations relating to the environment.
In addition, the application of varnishes to articles of complex shape (for example vehicular rear lights) does not lend itself easily to the production of layers of uniform thickness.
In contrast, layer deposition by plasma polymerization permits satisfying this requirement on an article of such type; indeed, the article "bathing" in the gaseous atmosphere of the plasma is accessible to gas molecules in a state of excitation at each of its points, insofar as the fluid mechanics of the system have been properly understood.
However, from the very outset of the development of the plasma deposition technique, problems have evolved relating to the adequacy of the adhesiveness of the thin layer to the plastic substrate onto which it is deposited and many attempts have since been made to obviate these disadvantage and drawbacks.
U.S. Pat. No. 4,830,873 describes a process for the deposition of a scratch-resistant "hard" film, preferably onto polycarbonate, via plasma polymerization, comprising first forming a prelayer from the monomer to be polymerized into a downstream final layer, and then incorporating oxygen progressively (for a few minutes) until the required proportions in the gas mixture have been attained that provide the "hard" film. According to this '873 patent, when oxygen is introduced at the beginning of the process, it is detrimental and adversely affects the subsequent adhesiveness between the hard layer and the substrate.
EP-A-0,254,205 describes a process for the deposition of transparent protective layers based on siliceous compounds, preferably on polyallyl diglycol carbonate. The plasma employed is based on hexamethylidisiloxane (HMDS) and oxygen and is obtained in a radio frequency reactor provided with capacitive coupling. The substrate is arranged on the electrode connected to the frequency generator (or cathode).
Certain specific plasma deposits exhibiting proper adhesiveness are also known to this art, the deposition of which principally being onto polycarbonate.
Such processes, however, do not provide satisfactory results in the case of polymethyl methacrylate and need continues to exist to further improve the adhesiveness of these protective layers to other plastic substrates.